The production of olefins, in particular ethene, is in general achieved by pyrolyzing petroleum hydrocarbon feedstocks such as gas oil obtained from an atmospheric distillation column, naphtha, natural gas liquids, butane, propane or ethane. Some crackers can use even heavier feedstocks, such as gas oil obtained from a vacuum distillation column.
Pyrolysis is also called steam cracking and comprises thermal cracking of hydrocarbons in the presence of dilution gas. The process comprises a convection zone, a cracking zone, a cooling zone and a separation zone. The pyrolysis furnace comprises the convection zone and the cracking zone. The convection zone comprises a first preheating zone and a second preheating zone. Generally, feed is heated in the first preheating zone, and dilution gas is added to the feed before the mixture of feed and dilution gas is sent to the second preheating zone. If the feed is liquid, generally at least part of the feed is vapourized in the first preheating zone.
It is well known that coke deposits in the cracking zone during operation. This coke layer inhibits heat transfer from the heating means in the cracking zone, it raises the wall temperature and it reduces the cross-sectional flow area of the tube. Because of the coke formation in the cracking zone, the pyrolysis furnace must be regularly shut down to remove the coke.
Generally, an olefin plant for converting liquid feeds will contain several pyrolysis furnaces of which the majority is designed for pyrolyzing liquid feed and generally a single furnace is designed for pyrolizing gaseous feed. The majority of the gaseous feed is usually produced in the olefin plant itself. If a pyrolysis furnace designed for gaseous feed is taken out of service, the feed needs to be processed in another pyrolysis unit. As there is generally not sufficient spare capacity for pyrolyzing gaseous feed, the gaseous feed generally is sent to a pyrolysis unit designed for pyrolyzing liquid feed. However, only a relatively small amount of gaseous feed can be sent to a furnace designed for liquid feed. The narrower, longer tubes in the convection zone designed for liquid feed, cause a larger pressure drop. Therefore, in some instances, part of the gaseous feed is added to fuel gas when a pyrolysis furnace designed for gaseous feed is out of operation. Further, it is disadvantageous to process gaseous and liquid pyrolysis feed in a single pyrolysis furnace as the process conditions for the feeds differ.
Liquid feeds which can be subjected to pyrolysis comprise gas oil and naphtha. A gas oil generally has a higher initial and final boiling point than naphtha. Furnaces designed for treating a heavy feed will have a larger heat transfer surface area in the first preheating zone than furnaces designed for light feed as a heavy feed has a higher initial boiling point than light feed and the main aim of the first preheating zone is vapourizing (part of) the feed and heating the feed. If a light feed such as naphtha is fed to a furnace designed for pyrolysis of gas oil, the naphtha will generally be fully vapourized when having passed part of the first preheating zone. If the amount of light feed sent to the pyrolysis furnace would be the same as the amount of heavy feed for which the furnace was designed, there would be an unacceptable large pressure drop over the first preheating zone as a large amount of gas would flow through the first preheating zone. Therefore, only a relatively small amount of light feed can be treated in a furnace designed for a heavy feed.
A furnace designed for treating gaseous feed will have a smaller heat transfer surface area in the first preheating zone than a furnace designed for liquid feed as a gaseous feed does not need to be vapourized.